Magnetic viscosity in the high frequency regime

The theory of magnetic viscosity is extended to the high frequency regime, when the relaxation time of the magnetization tau approaches 1/f(o), the inverse of the pre-exponential factor of the Neel law. Analytic expressions are derived for the coefficient of magnetic viscosity, irreversible susceptibility and fluctuation held of an aligned system of non-interacting moments. The theory is then applied to coherently rotating moments using Brown's asymptotic expression for the relaxation time. The variation of the prefactor f(o) is shown to result in enhancement of the magnetic viscosity and of the fluctuation field when the energy of activation E-B < 5kT. The results appear to be in good qualitative agreement with recent experimental data from pulsed-held magnetometry on systems that are more complex, such as magnetic recording media. The implications for the high frequency switching of the magnetization are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.